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Hindawi Publishing CorporationThe Scientific World JournalVolume
2013, Article ID 915237, 6
pageshttp://dx.doi.org/10.1155/2013/915237
Research ArticlePhotoresponsive Wettability in Monolayer Films
fromSinapinic Acid
Cleverson A. S. Moura, Douglas J. C. Gomes, Nara C. de Souza,
and Josmary R. Silva
Grupo de Materiais Nanoestruturados, Universidade Federal de
Mato Grosso, 78600-000 Barra do Garças, MT, Brazil
Correspondence should be addressed to Josmary R. Silva;
[email protected]
Received 20 August 2013; Accepted 19 September 2013
Academic Editors: D. Devine and D. Hua
Copyright © 2013 Cleverson A. S. Moura et al. This is an open
access article distributed under the Creative Commons
AttributionLicense, which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is
properlycited.
Sinapinic acid is an interesting material because it is both
antioxidant and antibacterial agent. In addition, when illuminated
withultraviolet light, it can exhibit the so-called
photodimerization process. In this paper, we report on the
investigation of monolayerfilms from
3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SinA)
deposited onto poly(allylamine hydrochloride), PAH,films. SinA
monolayers were prepared by using the layer-by-layer (LbL)
self-assembly technique. Adsorption kinetics curveswere well fitted
by a biexponential function suggesting that the adsorption process
is determined by two mechanisms: nucleationand growth of
aggregates. By using wetting contact angle analysis, we have found
that SinA monolayers exhibit photoresponsivewettability under UV
irradiation (365 nm); that is, wettability decreases with
increasing UV irradiation time.The photoresponse ofwettabilitywas
attributed to photodimerization process.This hypothesis was
supported by the dependence of surfacemorphologicalstructure and
absorption on UV irradiation time. The mechanism found in the
well-known transcinnamic acid crystals is used toexplain the
photodimerization process in SinA monolayers.
1. Introduction
Photoresponsive materials are an interesting class of newsystems
due to their potential application in devices suchas
microelectromechanical systems—MEMS [1]. In thesesystems, the
control of properties such as wettability viaone external stimulus
is a key requisite for their application.In general, temperature
[2], electric field [3], and light [4]have been used as stimulus
for the wettability control inmaterials. One interesting material
family, which responds toultraviolet (UV) radiation, is the named
cinnamic acid andits derivatives, which are widely used as model
systems forphotochemical reactions that can occur in condensate
phase[5]. When the molecules of these materials—arranged inparallel
stacking geometry—are exposed to ultraviolet light,they can undergo
crystalline structure transformation as aresult from
photodimerization process [5].This latter mecha-nism can cause
morphological changes and therefore leadingto a structural control
of the films. A lot of experimentaltechniques have been employed to
investigate films fromcinnamic acid derivates under UV irradiation,
for instance,
UV absorption [6], infrared [7], Raman spectroscopy [8], X-ray
structural analysis [9], and atomic force microscopy [10].In
particular, photodimerization has been studied on sinap-inic acid
by using subpicosecond time-resolved fluorescencespectroscopic
[11]. Moreover, SinA has been investigated as achromophore isolated
model for photoactive yellow protein(PYP). In this study, the
authors observed an unrelaxedground-state intermediate in
pump-probe signals by meansof pump-dump probe spectroscopy [12].
All these studies arecarried out in liquid or solid state phases;
however there arenot reports of studies of films prepared by
layer-by-layer self-assembly technique. The use of the LbL
technique can beinteresting because it allows surface structure and
thicknesscontrol, which leads to the buildup of the desired
systems[13, 14].
In this paper, we report on the preparation and inves-tigation
of the photoresponse of wettability of SinA mono-layers films
irradiated by ultraviolet radiation (365 nm).For these studies, we
have employed wetting contact anglemeasurements, atomic force
microscopy, and UV-visiblespectroscopy. SinA monolayers were
deposited onto PAH
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2 The Scientific World Journal
HO
H3CO
H3CO
H
H
HH
O
OH
SinA
(a)
CH2
CH2
n
NH+3 Cl−
PAH
(b)
Figure 1: Chemical structures of sinapinic acid (SinA) and
poly(allylamine hydrochloride) (PAH).
layers, which were deposited previously on quartz substrate.PAH
was used as a support because SinA molecules do notadsorb on bare
quartz.
2. Materials and Methods
3,5-Dimethoxy-4-hydroxycinnamic acid (sinapinic acid,SinA) was
purchased from Acros Organics (Figure 1).Poly(allylamine
hydrochloride) (PAH) (MW ∼15,000) waspurchased from Sigma-Aldrich.
All compounds were used asreceived.
For the adsorption kinetics experiments, the PAHmono-layers were
assembled by the immersion of the quartzsubstrate into PAH aqueous
solution (0.5mg/mL) for 3min.Following, the system (PAH monolayer
film + quartz sub-strate) was immersed into solution of SinA with
methanol(10.0mg/mL) for different immersion times (2–85 s) at
roomtemperature (20∘C). The immersion time is given by theaddition
of the times at each immersion step from thebeginning of the
experiment at 0 s. After each differentimmersion time, the SinA
monolayers were dried under anair flow and their absorbance was
measured by UV-visiblespectroscopy spectrophotometer (Thermolab,
Genesys 10).For the solutions of PAH, the pH was adjusted to 7.5
byadding NH
4OH. For the experiment of irradiation of films
with ultraviolet radiation, analyzes of AFM and for
wettingcontact angle, an immersion time of 45 s was used to
preparethe samples. It should be noted that we call each of our
filmsmonolayer because they are formed by the same materialin spite
of using various steps of deposition with differentimmersion times
to buildup them.
The exposures of the films to ultraviolet radiation werecarried
out by placing the samples into chamber with aPhilips TL UV mercury
lamp (6W, 365 nm). The films werepositioned 10 cm from the lamp.
The surface morphology ofthe monolayers was studied with a NanoSurf
Instrumentsatomic force microscope EasyScan ΙΙ in the tapping
mode(256 × 256 pixels) under ambient conditions. A sample areaof
10𝜇m × 10𝜇m was scanned and an image was acquired.The monolayer
roughness and aggregate average height anddiameter were determined
using NanoSurf Instrumentssoftware. Wetting contact angles were
measured with ahomemade instrument in ambient conditions. Purified
waterdroplets (volume of 5.0𝜇L) were gently placed onto the
film
0 10 20 30 40 50 60 70 80 90
0.000.060.01 0.01
0.020.030.040.050.060.070.080.090.10
Immersion time (s)
−0.01
Abso
rban
ce at
315
nmAbs = A
A
(1 − exp( t𝜏1
)) + B
B
(1 − exp( t𝜏2
))
0.9 ± 0.1
𝜏1(s) 𝜏2(s)
10.3 ± 3.6
Figure 2: Growth kinetics followed by absorbance at 315 nm
versusimmersion time for a SinA monolayer.
surfaces and the average values measured at six
differentlocations of each sample were taken. In order to found
theapparent surface energy, 𝛾tot
𝑠
, of themonolayers, we have usedthe following relation:
𝛾
tot𝑠
=
𝛾
𝑙(1 + cos 𝜃adv)
2
2 + cos 𝜃rec + cos 𝜃adv,
(1)
where 𝛾𝑙is the surface tension of water, 𝜃adv is the contact
angle of advance, and 𝜃rec is the receding contact angle[15].
Design and geometry optimization of SinA molecularwere carried out
in vacuum using MNDO method [16]implemented in ArgusLab 4.1
software [17].
3. Results and Discussion
3.1. Adsorption Kinetics. Figure 2 depicts the
adsorptionkinetics for SinA monolayer films. UV-vis spectra of
SinApresent a well-defined absorption bands around 315 nm,which is
attributed to 𝜋 → 𝜋∗ transition in aromatic rings[18].This value of
absorption peak was used to the adsorptionkinetics experiments.
The absorbance shows an increase as function of time anda
plateau which is observed at constant time of ca. 10 s.
Thissuggests that a whole monolayer was formed after 10 s. In
this
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The Scientific World Journal 3
(a) (b) (c)
Figure 3: Water droplets on SinA monolayer during the advance
wetting angle measurements: (a) as-prepared, (b) irradiated for 15
h, and(c) irradiated for 24 h.
Table 1: Values of wetting contact angles and apparent
surfaceenergy for as-prepared SinA monolayer and under different
UVirradiation times.
Time (h) 𝜃av (degree) 𝜃rec (degree) 𝛾𝑠 (mJ/m2)
0 42.2 34.6 35.615 72.3 37.8 30.724 100.1 43.7 23.6
situation, the available sites for SinA molecules adsorptiononto
PAH films from solution are repelled by those alreadyadsorbed with
the same charge.
The adsorption kinetics curve was fitted by an associ-ated
biexponential equation (inset Figure 2), where Abs isthe
absorbance, A and B are constants, 𝜏
1and 𝜏
2are the
characteristic times, and 𝑡 is the time [19].Associated
biexponential functions are commonly used
for fitting kinetics processes such as photoinduced
birefrin-gence [20]. In general, this function represents two
mecha-nisms in a kinetics phenomenon (fast and slow, resp.). Forour
results, the fast mechanism can be attributed to a fastadsorption
mechanism in which the molecules near the PAHmonolayer surface
diffuse towards it filling the adsorptionavailable sites.The
slowmechanism arises from small amountof available sites after a
few times and due to electrostaticrepulsion between adsorbed
molecules and those ones insolution [21].
3.2. Photoresponsive Wettability. In order to investigate
thephotoresponse of wettability from SinAmonolayer
underUVirradiation, we have performed wetting contact angle
andobtained surface energies of SinAmonolayers. Figure 3 showswater
droplets on a SinA monolayer, which were exposedto UV irradiation
at 0 (as-prepared monolayer), 15, and24 h. Table 1 displays the
values of wetting contact angles(advancing and receding) as well as
the apparent surfaceenergy values determined.
It is observed from Figure 3 and Table 1 that the wettingcontact
angles increases with increasing UV irradiation time,whereas the
apparent surface energy decreases.These findingindicates that the
monolayer surface structure is convertedfromWenzel one (Figure
3(a)) to a Cassie (Figure 3(c)) [22].
Table 2: Mean diameter, root-mean-square (RMS) roughness forSinA
monolayer at different times of UV irradiation.
Time (h) RMS roughness (nm)0 0.5815 1.4724 4.54
It is important to address that wetting contact angle andalso
apparent surface energy are determined by the chemicalcomposition
and surface morphological structure of materialsurface, which is
associated to its roughness [19, 23]; thehigher the roughness the
lower the wettability, that is, lowsurface energy. On the other
hand, photodimerization isassociated to structural changes of
material [11].Then, we canhypothesize that the photoresponsive
wettability exhibited bythe SinA monolayer under UV irradiation,
could occur dueto a photodimerization process, which leads with
structuralalterations and, consequently, to a higher roughness of
SinAmonolayers.
3.3. Surface Morphological Structure Analysis. In order
toexamine the hypothesis that increasing the roughness
coulddecrease the wettability of SinA monolayers, we have
carriedout atomic force microscopy analysis. Figure 4 shows theAFM
image of a SinA monolayer film AFM images for (a)PAH monolayer onto
quartz substrate, (b) SinA monolayerfilm onto PAH monolayer without
UV irradiation (c) SinAmonolayer after UV irradiation for 15 h, and
(d) SinA mono-layer after UV irradiation.
As shown in the Figure 4, we can observe that the
surfacemorphological structure is formed by rod-shaped
aggregates.This finding was expected since structural changes due
tophotodimerization have been observed for monolayers of4-(amyloxy)
cinnamic acid deposited on Au substrate [24].AFM images reveal that
the PAH film surface is very smooth,with roughness ca. 1 nm and
aggregate free (Figure 4(b)).Then, the contribution of surface
morphology structure ofPAH film surfaces on SinA monolayer
morphological struc-ture could be ruled out. From Table 2, we
observe that RMSroughness increase with increasing UV irradiation
time.This
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4 The Scientific World Journal
Z: 16.5nm
X: 5𝜇m Y: 5𝜇m
(a)
Z: 14.3nm
X: 5𝜇m
Y: 5𝜇m
(b)
Z: 20.2 nm
X: 5𝜇m
Y: 5𝜇m
(c)
Z: 73nm
X: 5𝜇m Y: 5𝜇m
(d)
Figure 4: AFM images for (a) PAHmonolayer onto quartz substrate,
(b) SinA monolayer film onto PAHmonolayer without UV irradiation(c)
SinA monolayer after UV irradiation for 15 h, and (d) SinA
monolayer after UV irradiation for 24 h.
result supports the hypothesis of increases in the roughnessof
SinA monolayer as the origin of the decreasing wettabilityof SinA
monolayer (Table 1) and the transition from Wenzelsurface to Cassie
one.
3.4. UV-Vis Analysis. In order to corroborate the
photodim-erization hypothesis of SinA under UV irradiation, we
havecarried out analysis of UV-vis spectroscopy. Figure 5 showsthe
UV-visible spectra for SinA monolayer, which weresubmitted to
different UV irradiation times. We can observea decrease of
absorbance as a function of the irradiation time.This behavior was
also noted by Davaasambuu et al. [25]being attributed to a decrease
in monomer concentration asa result from photodimerization
process.
Our result is consistent with those found from AFM(Section 3.3)
and wettability contact angle analysis(Section 3.2). Therefore, it
is reasonable suggesting that aphotodimerization process is
responsible by the photo-response of wettability of the SinA
monolayer under UVirradiation.
3.5. Photodimerization Mechanism. In a study of self-assembled
monolayers from 4-(amyloxy)cinnamic acid, Xuet al. [24] suggested
that a photodimerization can arise from
225 250 275 300 325 350 375 400 425 450 475 500 525
Abso
rban
ce
Wavelength (nm)
365 nm
24 h
As-prepared (0 h)
15 h
Figure 5: UV-visible spectra for as-prepared SinA monolayer
andirradiated by UV radiation (365 nm) for 15 and 24 h.
a molecular photoexcitation, which leads to a short-term
lat-tice instability.This processwould place onemolecule close toa
neighbor producing amore favorablemolecular orientationand then
leading to photodimerization reaction. Unfortu-nately, our results
do not allow stating if this mechanism
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The Scientific World Journal 5
(a)
(b)
Figure 6: Possible molecular structures of dimmers formed
fromphotodimerization of SinA from (a) head-tail arrange and (b)
head-head arrange. Grey balls represent carbons, red balls
representoxygens, and white balls represent hydrogens. The
structures wereoptimized by using the MNDOmethod.
occurs in the SinA monolayer. On the other hand, similarlyto
photodimerization, which occur in transcinnamic acid [7],we can
suggest two possible forms of dimer which can occurin the SinA
monolayers after UV irradiation (a) head-headand (b) head-tail, as
showed in Figure 6.
Our results still do not allow stating which dimer formoccur
after UV irradiation of the SinA monolayer. A way ofclarifying this
point would be using the method proposed byAtkinson et al., which
is based on vibrational spectroscopy[7].
4. Conclusion
We have prepared for the first time SinA monolayer filmsusing
PAH monolayer films as a support layer. Thoughadsorption kinetics
experiments, the immersion time ofsaturation was found to be ∼10 s.
In addition, we have foundthat the growth process consists of two
mechanisms, a fastone and another slow, which were associated with
a fastadsorption limited by diffusion and an electrostatic
repul-sion, respectively. We have found that the wetting
contactangle of SinA films increases with increasing UV
irradia-tion time. This suggests that a photodimerization
process
plays an important role in the photoresponse of wettability.This
hypothesis was corroborated by the surface morpholog-ical structure
changes anddecreasing in electronic absorptionobserved for the SinA
monolayers as a function of irradi-ation time. In summary,
monolayers films from cinnamicderivates—prepared by the LbL
self-assembly technique—may be useful for future studies not only
on photoresponseof wettability but also on elementary ablation,
ionizationprocesses.
Acknowledgment
The authors acknowledge the financial support from CNPqand CAPES
(Brazil).
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